feat: return-to-door system for map exits

When entering an interior (e.g. lab from town), the game saves the
player's current position as a return point. When exiting, if the
exit has no explicit targetX/targetY, the system pops the stored
return point and warps back to that exact position.

This means interior exits just need targetMap — the player always
returns to the specific door they entered from, not a hardcoded
position. Falls back to the destination map's spawn if no return
point is stored.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

js/world/worldState.js

@@ -40,6 +40,10 @@ export const worldState = {
     solvedPuzzles: [],  // array of puzzleIds that have been solved
     activePuzzle: null,  // { puzzleId, requiredOutputs, doorX, doorY } or null when no puzzle active
 
+    // Return points — remembers where the player entered each map from
+    // Stack of { fromMap, fromX, fromY } — push on enter, pop on exit
+    returnPoints: [],
+
     // Game flags
     flags: {
         // Examples: